Prevention of photoresist scumming

ABSTRACT

A photo acid generator (PAG) or an acid is used to reduce resist scumming and footing. Diffusion of acid from photoresist into neighbors causes a decreased acid level, and thus causes resist scumming. An increased acid layer beneath the resist prevents acid diffusion. In one embodiment, the increased acid layer is a layer of spun-on acid or PAG dissolved in aqueous solution. In another embodiment, the increased acid layer is a hard mask material with a PAG or an acid mixed into the material. The high acid content inhibits the diffusion of acid from the photoresist into neighboring layers, and thus substantially reduces photoresist scumming and footing.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/856,556, filed Sep. 17, 2007, which is issued as U.S. Pat. No.7,704,673 on Apr. 27, 2010; which is a divisional of U.S. applicationSer. No. 11/471,012, filed on Jun. 20, 2006, which is issued as U.S.Pat. No. 7,270,917 on Sep. 18, 2007; which is a divisional of U.S.application Ser. No. 10/940,805, filed on Aug. 31, 2004, which issued asU.S. Pat. No. 7,175,944 on Feb. 13, 2007. Each of the priorityapplications is hereby incorporated by reference in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the field of integrated circuitfabrication, specifically to the field of microlithography.

2. Description of the Related Art

Modern integrated circuits are getting more dense and compact. Toaccommodate smaller features, new photoresists are being used.Currently, UV wavelengths of 248 nm and 193 nm are being used formicrolithography tools. Soon, the exposure will be carried out at evenlower wavelengths, including 157 nm. These low wavelengths will allowfor the use of sub-70 nm features. To remove photoresist, a chemicaldistinction is provided between exposed and unexposed resist. This isprimarily done through the use of photo acid generators (PAG),photoactive materials that form an acid upon UV exposure at the properwavelength. That acid buildup allows for selective removal of exposedphotoresist, while leaving unexposed resist in place.

A problem with small photoresist wavelengths arises from interfacialinteractions between the resist and the underlying layers. One suchproblem is that the acid generated upon exposure diffuses intounderlying layers. Because some of the acid has diffused out of theresist, sufficient acid may not remain to facilitate the removal of allof the exposed resist. Some of the exposed resist is then not removedfrom the surface of the underlying layer, leaving a “resist scum” thatcan cause fabrication failures. The resist scum can cause footing alongthe edges of the exposed photoresist, decreasing the width of theopenings in the pattern. Ultimately, the scumming or footing of theresist on the surface of the underlayers could change the criticaldimensions (CD) of the device. CD is defined as the dimensions of thesmallest features (e.g., width of interconnect line, contacts, trenches,etc.) defined by photolithography in the course of semiconductordevice/circuit manufacturing using a given technology.

Recently bottom anti-reflective coating (BARC) has been used beneath lowwavelength photoresist. However, in some circumstances, the use of aBARC is impracticable due to integration issues. For example, when theBARC layer is organic, the etchant that removes the BARC can possiblydamage any underlying organic layer.

Some methods have been tried to combat the diffusion problem with shortwavelength photoresist. One such method is to put a thermal acidgenerator (TAG) into a BARC material. One example of this is U.S. Pat.No. 6,329,117, issued to Padmanaban, et al. When the TAG is heated, itcreates an acid in the BARC. This higher acidity level discourages aciddiffusion. However, this method usually requires a post-exposure bake(PEB) or other heating process. Additionally, in some fabricationprocesses, a BARC layer is not desired between the photoresist layer andan underlayer.

U.S. Pat. No. 6,528,235 issued to Thackeray et al. on Mar. 4, 2003describes an anti-reflective coating layer, called an anti-halationlayer, with an acid additive, such as acid generators and acids. Thelayer can be removed during resist stripping and easily etched throughwhen etching the underlying substrate. The optional acid additive isadded to catalyze the crosslinking reaction between a resin binder andthe crosslinker. The anti-halation layer bonds to the photoresist duringa thermal process.

Each of the above methods faces difficulties and disadvantages.Therefore, there is a need for methods and procedures for the preventionof resist footing and scumming.

SUMMARY OF THE INVENTION

In one aspect of the invention a method of forming a hard mask withimproved pattern transfer is disclosed. The method comprises forming afirst hard mask layer with an acid additive over a substrate. Aphotoresist layer is deposited over the first hard mask layer. Thephotoresist layer is patterned to form a pattern. The pattern istransferred to the first hard mask layer after the photoresist layer ispatterned.

A method for forming a hard mask is disclosed in another aspect of theinvention. The method comprises forming a first hard mask layer over asubstrate and forming an acid layer over the first hard mask layer. Theacid layer consists essentially of an acid and a diluting agent. Aphotoresist layer is deposited over the acid layer. The photoresistlayer is patterned to form a pattern. The method further comprisestransferring the pattern to the first hard mask layer below the acidlayer.

In another aspect of the invention, a chemical precursor for a hard maskto prevent resist scumming is disclosed. The precursor comprises aspin-on dielectric deposition (SOD) precursor. The SOD precursor isselected to form an inorganic layer after solidifying. The precursorfurther comprises an acid additive.

In another aspect of the invention, a method of forming a hard maskresistant to photoresist scumming is disclosed. A spin-on deposition(SOD) precursor containing a photo acid generator is deposited over asubstrate. The SOD precursor is solidified to form a first hard masklayer. A photoresist layer is deposited over the first hard mask layer.A pattern is exposed in the photoresist layer. The pattern in thephotoresist layer is then developed. The pattern is transferred to thefirst hard mask layer after developing the pattern. The photoresistlayer is then removed after transferring the pattern to the first hardmask layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic cross-sectional side view of a hard maskpatterning structure.

FIG. 1B is a schematic cross-sectional side view of the hard maskpatterning structure of FIG. 1A after exposure to UV radiation.

FIG. 1C is a schematic cross-sectional side view of the hard maskpatterning structure of FIG. 1B after developing the resist pattern.

FIG. 2A is schematic cross-sectional side view of a hard mask patterningstructure according to a preferred embodiment.

FIG. 2B is a schematic cross-sectional side view of a hard maskpatterning structure according to another preferred embodiment.

FIG. 3 is a schematic cross-sectional side view of the hard maskpatterning structure of FIG. 2A after photoresist patterning.

FIG. 4 is a schematic cross-sectional side view of the hard maskpatterning structure of FIG. 3 after transferring the resist pattern toa first hard mask.

FIG. 5 is a schematic cross-sectional side view of the hard maskpatterning structure of FIG. 4 after transferring the pattern to a lowerlayer.

FIG. 6 is a schematic cross-sectional side view of the hard maskpatterning structure of FIG. 5 after a substrate etch process.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The significance of the problem of photoresist scumming and footing isgrowing because of the decreasing size of the resist and the circuitsthat are being patterned. Scumming, or resist remaining afterdevelopment, is at least partially the result of resist that was notfully broken down by the acid generated in the resist. This scumming cancause footing and poorly formed features.

FIGS. 1A-1C illustrate a typical photoresist exposure and developingprocess and scumming effects. FIG. 1A shows a reticle 10 spaced above aphotoresist layer 20. The photoresist 20 is over a first hard mask layer30, a lower layer 40, and a substrate 45. The first hard mask layer 30,in this case an insulating underlayer such as an oxide, is directlybeneath the resist 20. An illustrated lower layer 40 beneath the firsthard mask layer 30 can be the substrate to be processed or an additionalhard mask material. In the illustrated embodiment, the lower layer 40comprises a layer of amorphous carbon 40 beneath the first hard masklayer 30. The substrate 45 is shown beneath the lower layer 40. Thesubstrate 45 can represent any structure to be processed (e.g., etched)through the hard mask to be patterned. The resist 20 is exposed to UVradiation 8, thereby producing acid in the resist that allows thedeveloping and removal of the resist 20. Using the reticle 10 is placedover the resist, only the desired portion of the resist will be exposedand subsequently developed.

FIG. 1B illustrates the acid generation in the exposed photoresist area23. Acid is generated in the exposed area 23 when the photo acidgenerator (PAG) is exposed to UV radiation. Problems can arise in thedeveloping of photoresist if the acid generated during exposure is notevenly distributed throughout the exposed area 23. However, in theillustrated embodiment, acid diffuses to neighboring layers that areless acidic than the exposed photoresist region 23. The diffusion ofacid causes an acid deficiency around the edges of the exposedphotoresist region 23. In the illustrated embodiment, the first hardmask layer 30 is directly beneath the photoresist. Many insulators haveacid concentration that is substantially lower than exposed photoresist.The difference of the acidity level between the two neighboring layersencourages the diffusion of acid from the exposed photoresist area 23into the first hard mask layer 30.

As illustrated in FIG. 1C, the exposed photoresist area 23 of a positivephotoresist layer 20 has been removed by a developer chemical. Becauseof acid diffusion to the first hard mask layer 30, the photoresist hasleft a scum around the edges of the exposed area. As illustrated, thephotoresist has left behind a corner footing 16 and a central scum layer18 over the first hard mask layer 30. These scum layers can cause poorpattern transfer from the photoresist layer into the first hard mask.

The diffusion of acid into an underlayer can cause deviation from theintended pattern. Photoresist is used to pattern metal lines, hardmasks, and other layers. The problems arising from acid diffusionbetween resist and neighboring layers can cause non-working componentsand connections, such as faulty transistors, disconnected metal lines,short circuits and associated problems. If proper patterning is notconsistently achieved, lower yields result. This, in turn, increasesoverall production costs.

Process Flow

An exemplary process flow of the hard mask is seen in FIGS. 2-6. In FIG.2A, the structure is shown being exposed to radiation 8, preferably UVradiation with a wavelength of a common lithography node (e.g., 157 nm,193 nm, and 248 nm). The exposure of the photoresist 20 to UV radiationactivates the photo acid generator (PAG) in the photoresist. In apreferred embodiment, a first hard mask layer 32 in accordance with apreferred embodiment is situated over a second hard mask layer 40 and asemiconductor substrate 45.

FIG. 2A illustrates the patterning of the photoresist layer 20 using areticle 10, similar to the process of FIG. 1A. In a preferredembodiment, the lower or second hard mask layer 40 is a carbon layer,preferably amorphous carbon that can be used as a hard mask for etchingthe substrate 45. Amorphous carbon can be formed by several depositionprocesses, including chemical vapor deposition (CVD), sputtering, andion beam deposition. The first hard mask layer 32 is preferablydeposited by spin-on deposition (SOD).

In the illustrated embodiment of FIG. 2A, the photoresist layer 20 ispreferably a positive photoresist, but negative photoresists can also beused for the photoresist layer 20. In a typical positive resist, aprotective chemical that is affected by acid, rather than by heat orwater, is used to protect the photoresist from the resist developer.Several common protective chemicals are used to mask the basesolubilizing hydroxyl functionality of a polymer. Exemplary chemicalsinclude a carbonate [e.g., t-boc (t-butoxycarbonyl)], ester, ether,acetal and ketal. A photoacid generator (PAG) generates an acid moleculeand catalytically degrades the resist to a form susceptible todissolution in the developer by breaking down a protective chemical onthe resin in the light exposed regions during a post-exposure step, suchas a post-exposure bake (PEB). While this reaction will take place veryslowly at room temperature, it is much faster at PEB temperatures(60-150° C.), requiring only a few seconds to reach completion. In theabsence of an acidic species, the protected polymer undergoes nodegradation during prolonged heating at the PEB temperature.

However, if the acid generated by UV exposure in the photoresist layer20 diffuses to an underlayer, such as the first hard mask layer 32, theresist layer will not be properly developed. In order to correct aciddiffusion problems in accordance with the preferred embodiments, acid isincreased in a layer below the exposed photoresist layer 20. Due to thedependence of diffusion upon relative acid concentrations, acid willdiffuse from the more acidic layer to the less acidic layer. Inaccordance with the preferred embodiments, the acid level is increasedbeneath the photoresist layer 20 to correct acid diffusion problems. Inan embodiment of the acid diffusion prevention process illustrated inFIGS. 2A and 3-6, a photo acid generator material is dissolved into aspin-on precursor material used for the first hard mask layer 32 inorder to increase the acidity below the exposed resist layer 20. In anembodiment illustrated in FIG. 2B, an acid layer 25 is deposited tointervene between the photoresist layer 20 and first hard mask layer 32.These preferred methods of increasing the acidity of the underlayer willprevent many interfacial interaction problems between the photoresistlayer 20 and the first hard mask layer 32.

First Hard Mask Layer

In a preferred embodiment, the acid concentration of the first hard masklayer 32 is coordinated with the photoresist layer 20. In oneembodiment, the acidity level of a first hard mask layer 32, such as aspin-on deposition (SOD) dielectric material, is increased from typicallevels of underlayer materials. Preferably, this is performed byintroducing an acid additive, either an acid or an acid generator, intothe precursor material of the first hard mask layer 32 before it isdeposited over the substrate, although acid can alternatively be addedduring or after spin-on deposition and before curing.

In a preferred embodiment, an acid is mixed into the precursor of thefirst hard mask layer 32, preferably a spin on deposition dielectricprecursor. Many acids can be mixed into the insulator precursor, butpreferred acids to be used as the acid additive include organic acids,such as phosphoric acid (H₃PO₄), and hydrochloric acid (HCl). Preferredacid concentration levels of the SOD precursor with added acid isbetween about 0.005 mol/Liter and 0.1 mol/Liter. Skilled practitionerswill appreciate that many types of acids can be used in the dielectricmaterial to increase the acid concentration in the SOD precursor,without causing damage to neighboring layers.

Several types of spin on deposition dielectric materials can be used.Preferably, the SOD material is of a type that solidifies into aninorganic layer. The SOD material selected preferably produces a form ofsilicon-containing dielectric materials, such as silicon oxide, aftercuring. In one embodiment, a spin-on-glass (SOG) is used. SOGs arewidely used and are available from several manufacturers, including DowCorning, Inc. of Midland, Mich. and Clariant Life Sciences K.K. ofTokyo, Japan. Many of these films require a bake process, which makesthe high temperature stability of ionic PAGs helpful. A preferredinorganic class of SOD precursor materials is the class of silazanematerials, such as poly(perhydrosilazane) (SiH₂NH). These materials areavailable under the tradename Spinfil™ from Clariant Life Sciences K.K.The same company offers organic SOD materials, such asPoly(methylsil(sesqui)azane) (SiCH₃N_(1.5)), under the tradenameSigniflow™. Another preferred class of SOD materials are silsesquioxanebased materials. Two varieties of these products are available, organicand inorganic. Hydrogen silsesquioxane (HSQ) is a commonly availableinorganic SOD material and methyl silsesquioxane (MSQ) is a commoninorganic SOD material.

SOD materials typically are delivered with a manufacturer recommendedbaking, or solidifying, recipe. For example, the recommended bakingrecipe for one type of Spinfil calls for hot plate baking at about 150°C. for about 3 minutes, followed by furnace curing at between about 350°C.-800° C. for approximately 30 minutes in a steam ambient environment.Skilled practitioners will appreciate that there are several appropriatesolidifying techniques.

Once the precursor of the first hard mask layer 32 is deposited onto thesurface of the wafer and solidified to form a solid layer, thephotoresist 20 can be applied over the first hard mask layer 32. Duringthe exposure and developing of the photoresist 20, the acid generated byPAGs in the resist will be less likely to diffuse out because of theincreased acidity of the directly underlying first hard mask layer 32.The exposure and developing process will continue as in a standardphotoresist process, but reduced diffusion results in better patterningof the photoresist mask and subsequent layers.

In another preferred embodiment, a photo acid generator (PAG) is used toincrease the acidity of the material that is used for the first hardmask layer 32. Preferably, the PAG is mixed into a spin-on depositionprecursor material similar to the materials described above. When a PAGis exposed to UV radiation, the PAG forms an acid. Many PAG materialsare available in the form of a powder, which enables them to bedissolved into liquid SOD precursor materials either with or without theuse of a solvent such as propylene glycol monomethyl ether acetate(PGMEA). However, typically an SOD precursor material will contain asolvent that will dissolve a powder based PAG. For example, the Spinfil™SOD precursor material contains dibutylether (C₈H₁₈O), which candissolve many powder based PAG materials. Once the PAG is mixed into theprecursor of the first hard mask layer 32, the precursor is depositedonto the wafer, preferably by a spin-on deposition tool in the samemanner that it would ordinarily be processed. Because the consistency ofthe liquid precursor should not be substantially altered by the additionof the PAG powder, many spin-on tools are available for use in thisapplication.

There are two primary types of PAGs currently in use. First, neutral, ornon-ionic, PAGs are typically organic materials and are oftenaccompanied by the addition of a sensitization additive in order tobecome photoactive. Non-ionic PAGs such as phloroglucinyl ando,o-dinitrobenzyl sulfonates, benzylsulfones and some 1,1,1-trihalidesare more compatible with hydrophobic media in general, although theirthermal stabilities and quantum yields for acid generation are oftenlower.

However, most of the PAGs currently used in resists with associatedwavelengths of 248 nm, 193 nm, or 157 nm are ionic PAGs. Ionic PAGs aretypically salts comprising a photoactive cation, also known as thecounterion, and an anion. Many photoactive cations can be combined withseveral different anions to form a PAG. Unlike many neutral PAGs, ionicPAGs typically are capable of producing acid by direct UV irradiation(between 10 nm and 380 nm). The anion is typically not photosensitive,but it will usually determine the acid that is produced by the PAG. Thecation will usually act as the photosensitive component and break downupon UV exposure. Common cations for PAG salts include triarylsulfoniumand diaryliodonium. Salts using these cations have become common PAGingredients in 248 nm resist formulations, because of their generallyeasy synthesis, thermal stability, high quantum yield for acidgeneration, and the strength and non-volatility of the acids theysupply. Ionic PAGs generally have low volatility and high thermalstability, features that can be important during processing steps thatrequire applying heat, such as a pre-bake or a post-bake step.

In a preferred embodiment, the acid additive used in the SOD precursoris an ionic PAG. Several PAG anions and cation combinations can be usedin combination to form the PAG salt used in the first hard mask layer32. PAGs can often be dissolved in liquids such as a photoresist, or inthe case of the embodiments illustrated in FIGS. 2-6, a spin ondeposition precursor. Preferred ionic PAG anions include antimonyfluoride (SbF₆), and phosphorus fluoride (PF₆). These materialsrespectively react to form hydrogen hexafluoroantimonate (HSbF₆) andfluorophosphoric acid (HPF₆), respectively, upon UV exposure. Preferredcations include triarylsulfonium and diaryliodonium. Another preferredclass of PAG materials breaks down to form carboxylic acid, such asdiazoquinone PAGs. In one embodiment, the same PAG that is used in thephotoresist is also used in the first hard mask layer 32. However, otherPAGs available as a powder and subject to activation upon exposure tothe UV wavelength used for the photoresist will be sufficientlyeffective to increase the acidity of a resist underlayer upon UVexposure. Alternatively the PAG in the first hard mask layer 32 can beactivated by a separate exposure of a different wavelength. A skilledpractitioner will appreciate that a broad range of PAGs will be usefulin this context.

In a preferred embodiment, the PAG will be mixed into the SOD precursorbefore the spin-on deposition. Generally, a preferable percentage byweight of PAG in the SOD liquid precursor is between about 0.5% and 20%,more preferably between about 1% and 15% by weight.

In an exemplary embodiment, a PAG from the preferred class of PAGmaterials that breaks down to carboxylic acid is added to asilsesquioxane SOD precursor material. Between about 0.5 g and 1 g ofthe PAG are added to between about 100 g and 200 g of the liquid SODprecursor material to form a SOD precursor that will alleviate resistscumming. Preferably the ratio of PAG material to SOD precursor isbetween about 0.0025 and 0.01.

PAGs mixed into SOD precursors can also be used along with other hardmask materials. For example, a thin SOD layer with a PAG can be usedover a thicker TEOS (tetraethylorthosilicate) oxide layer, nitridelayer, or other hard mask material. Typically, these thicker hard maskswill be deposited by a CVD process.

In another embodiment illustrated in FIG. 2B and further explainedbelow, a thin layer of acidic material 25 is deposited between theresist 20 and the first hard mask layer 32. The acid layer 25 can bedeposited by conventional deposition methods suitable for depositingliquid precursor materials, such as SOD or misting. Preferred materialsfor the acid layer 25 include organic acids, such as carboxylic acids,phosphoric acid (H₃PO₄) and hydrochloric acid (HCl). While the followingprocess makes reference to the structure of FIG. 2A, the structure ofFIG. 2B can also be used in a similar manner.

FIG. 3 illustrates the structure of FIG. 2A after the photoresist layer20 is patterned. The photoresist is developed to remove the exposedphotoresist area 25. The resist material will be removed as masked withfewer scumming and footing problems on the first hard mask layer 32. Thedeveloping of the photoresist prepares the first hard mask layer 32 forpatterning. The first hard mask layer 32, which preferably contains theacid additive, can then be used to etch or otherwise process thesubstrate 45 or the pattern can be first transferred to the optionalsecond hard mask layer 40. The addition of the acid additive does notprevent the use of the first hard mask layer 32 as a hard mask forunderlayers.

In FIG. 4, the unexposed portions of the resist layer 20 have been usedas a soft mask to pattern the first hard mask layer 32. The first hardmask layer 32 is preferably patterned using an anisotropic etch usingetchants such as fluoride reactive species or bromide reactive species.Skilled artisans will appreciate that there are several methods ofetching the first hard mask layer 32.

The photoresist layer 20 can either be removed after etching the firsthard mask layer 32 or it can be removed during the pattern transfer tothe second hard mask layer 40. The removal of the photoresist layer 20is preferably selective to the first hard mask layer 32. There are awide variety of resist strippers available for this purpose. The skilledartisan can determine a proper resist stripper to remove the photoresistlayer 20 without substantially damaging the first hard mask layer 32,which is preferably resistant to conventional organic strippers, such asoxygen plasma or aqueous organic solvents.

FIG. 5 illustrates the removal of the photoresist layer 20 and apreferred step of transferring the pattern from the first hard masklayer 32 into the second hard mask layer 40. However, the first hardmask layer 32 can also be used to etch or otherwise process thesubstrate 45 directly. In a preferred embodiment, the lower layer 40 isa hard mask material, such as amorphous carbon, with a thickness ofbetween about 1,000 Å and 14,000 Å, more preferably between 2,500 Å and12,000 Å In a preferred embodiment, the carbon-based second hard masklayer 40 is etched using a O₂ and SO₂ based plasma. This plasma etchprocess provides excellent selectivity to the first hard mask layer 32.Such a process is described in an application entitled “CriticalDimension Control for Integrated Circuits” by Abatchev, et al., filed onAug. 31, 2004 (Attorney Docket No. MICRON.286A, Micron Reference No.03-1348.00/US). The disclosure of this application is incorporated byreference herein.

One preferred chamber for etching the carbon-based second hard masklayer 40 is Lam Research Corp.'s (Fremont, Calif.) TCP9400 poly etchchamber. In this chamber, the pressure is preferably between 3 mTorr and20 mTorr, more preferably between about 5 mTorr and 15 mTorr. Theionizing source power, preferably delivered in situ, is preferablybetween 175 W and 400 W, more preferably between about 225 W and 350 W.The bias power is preferably between about 25 W and 125 W, morepreferably between about 30 W and 100 W. The electrode temperature ispreferably targeted to be between about −5° C. and 15° C., morepreferably between about 0° C. and 10° C. Preferred precursor gassesinclude SO₂, O₂, Ne, and Ar. In a preferred embodiment with a singlewafer, the flow rate for SO₂ is preferably between about 10 sccm and 75sccm, more preferably between about 20 sccm and 60 sccm. The flow ratefor O₂ is preferably between about 10 sccm and 100 sccm, more preferablybetween about 20 sccm and 80 sccm. The flow rate for Ar is preferablybetween about 0 sccm and 175 sccm, more preferably between about 0 sccmand 140 sccm. As seen in FIG. 5, when the lower layer 40 is etched, theunexposed resist 20 may also be removed.

In the figures, the substrate 45 is pictured as a semiconductormaterial. However, the substrate can be any material that is processedusing a mask or a hard mask. Exemplary materials for the substrate 45include semiconductor materials, conductive metals, insulatinginterlevel or intermetal dielectrics (ILD or IMD), or other type ofintegrated circuit material.

FIG. 6 shows the etching of the substrate 45, preferably a semiconductorsubstrate. The substrate can be a number of materials, such as aninsulating layer or a metallization layer. A skilled practitioner willappreciate that there are several acceptable methods of etching thesubstrate with a wide variety of parameters and that etchants andsettings for etching the substrate 45 will vary based upon the etchingequipment and substrate material selected.

An Acidic Layer

In an embodiment shown in FIG. 2B, a thin acid-rich layer 25 isdeposited directly below a photoresist layer 20. The acid-rich layer 25is deposited before the photoresist 20 is deposited. The acid-rich layer25 prevents the acid diffusion between an under layer and thephotoresist 20. Preferably, the acid layer is deposited directly over afirst hard mask 30. The first hard mask layer 30 can be a spin-ondielectric layer, a plasma enhanced CVD silicon oxide or siliconoxynitride layer, a TEOS oxide layer, or any other suitable hard maskmaterial. Preferably the thickness of the first hard mask 30 is betweenabout 100 Å and 1000 Å, more preferably between about 150 Å and 500 Å.Skilled practitioners will appreciate that the acid-rich layer 25 can beused over several different materials for the first hard mask layer 30.

Preferably, the acid-rich layer 25 is deposited on the wafer as aliquid, and is dried before the deposition of resist 20. In a preferredembodiment, the precursor of the acid-rich layer is an acid diluted witha diluting agent, such as water or propylene glycol monomethyl etheracetate (PGMEA). In a preferred embodiment, the acid-rich layer 25 isdeposited onto the wafer by spin-on deposition or direct liquidinjection tools. An acid-rich liquid is dripped onto the wafer as aliquid, then the wafer is spun to distribute the material. In anotherembodiment, a dip and dry process is used to apply the liquid acidiclayer. In this embodiment, the wafer is dipped in an acid solution anddried to form the thin acid-rich layer 25. Preferably, the acid-richlayer 25 is very thin, between about 10 Å and 100 Å, more preferablybetween about 15 Å and 40 Å. Skilled practitioners will appreciate thatthere are several methods in which to apply the acid-rich layer 25.

Preferred materials include the general set of acids. More preferably,the acid layer is a form of phosphoric acid (H₃PO₄) or hydrochloric acid(HCl). Preferably, the acid is diluted so that neighboring layers arenot damaged by the acid. An acid layer 25 can be formed using HCl orH₃PO₄ with a concentration of acid of between about 0.01 mol/Liter and0.1 mol/Liter, more preferably between about 0.025 mol/Liter and 0.075mol/Liter. Other acids can also be used but depending on the etchingcapabilities of the acid, they will be more or less appropriate for theuse over the first hard mask layer 30.

A liquid PAG solution could also be used in place of the layer of acidas the acid rich layer 25. In a preferred embodiment, the liquid PAGsolution is a PAG diluted with a solvent such as water or PGMEA. Whenthe photoresist layer 20 above the dissolved PAG layer 25 is exposed toUV, the PAG in the acid rich layer 25 forms an acid which inhibits thediffusion of acid from the photoresist 20 into the first hard mask 30.Preferably, the liquid is applied either by a spin-on deposition processor by a dip-dry process as described above. The layer is also preferablyquite thin, having a thickness of between about 10 Å and 100 Å, morepreferably between about 15 Å and 40 Å. Preferred PAG materials areeasily soluble in an appropriate solvent, such as water or PGMEA. Theskilled practitioner will appreciate that several PAG materials andsolvents will satisfy the requirements for the thin PAG layer.

The provision of an acid or PAG layer under a resist layer is alsouseful in any context where resist scumming is a problem. However, it isimportant to note that the acid or PAG is preferably selected inconjunction with the selection of neighboring layers in order tominimize damage to the neighboring layers. The effects of scumming oftenrequire a special “de-scumming” step, the need and/or vigor of which canbe substantially reduced through the use of an acid diffusion preventionsystem of the present invention.

Structure

Preferred embodiments are used as hard masks for an etching process. Inone embodiment seen in FIG. 4, the second hard mask layer 40 is over asubstrate 45. The substrate 45 is pictured as a semiconductor layer inthis figure. The second hard mask layer 40 is preferably a thick layerthat can be used as a hard mask for etching the material below. In onepreferred embodiment, the second hard mask layer 40 is a carbon-basedmaterial, such as amorphous carbon. The second hard mask layer 40preferably has a thickness of between about 500 Å and 15,000 Å, morepreferably between about 1500 Å and 10,000 Å.

Over the second hard mask layer 40 is the first hard mask layer 32 whichcan be used as a hard mask for the second hard mask layer 40 (whenpresent) or for the substrate. In one embodiment, the dielectric layer32 is a spin-on deposition dielectric containing a PAG material or othersource of acidity. The acidic first hard mask layer 32 preferably has athickness of between about 100 Å and 1000 Å thick, more preferablybetween about 150 Å and 600 Å.

In another embodiment illustrated in FIG. 2B, an acid rich layer 25 isover a first hard mask layer 30. The acid rich layer 25 is preferablydeposited in the form of a standard liquid acid solution or as a liquidacid generator solution. Preferably, the acid rich layer 25 is betweenabout 10 Å and 100 Å, more preferably between about 15 Å and 40 Å.Preferred acids include H₃PO₄ and HCl. The acid rich layer 25 and thefirst hard mask 30 together are between about 120 Å and 1000 Å, morepreferably between about 200 Å and 600 Å. The second hard mask layer 40is similar to the lower layer as described with reference to FIG. 4. Thesecond hard mask layer 40 preferably has a thickness of between about500 Å and 15,000 Å, more preferably between about 1500 Å and 10,000 Å.

While the use of acid additives in a liquid spin-on dielectric precursoris described in the context of a hard mask, it can also be used in othercontexts. Specifically, it can be used wherever a photoresist layer isin direct contact with a hard mask layer that is susceptible todiffusion.

In addition to straight anisotropic pattern transfer, “transferring apattern” from one layer to another layer can also encompass interveningmodification of a mask pattern. For example, a soft mask can beprocessed in order to shrink or grow the feature size. The mask patterncan be altered prior to, after, or during transfer to the first hardmask layer 32 or the second hard mask layer 40.

Although this invention has been described in terms of a certainpreferred embodiment and suggested possible modifications thereto, otherembodiments and modifications may suggest themselves and be apparent tothose of ordinary skill in the art are also within the spirit and scopeof this invention. Accordingly, the scope of this invention is intendedto be defined by the claims which follow.

1. A method of forming an inorganic layer, comprising: providing aliquid spin-on dielectric (SOD) precursor with acid in it; applying theliquid SOD precursor to a substrate; and solidifying the SOD precursorto form an inorganic layer.
 2. The method of claim 1, wherein the SODprecursor has a concentration of acid between about 0.005 mol/Liter and0.1 mol/Liter.
 3. The method of claim 1, wherein the SOD precursorcomprises a silsesquioxane.
 4. The method of claim 3, wherein the SODprecursor comprises a hydrogen silsesquioxane.
 5. The method of claim 3,wherein the SOD precursor comprises a methyl silsesquioxane.
 6. Themethod of claim 1, wherein the SOD precursor comprises an inorganiccompound.
 7. The method of claim 6, wherein the SOD precursor comprisesa silazane compound.
 8. The method of claim 7, wherein the SOD precursorcomprises poly(perhydrosilazane).
 9. The method of claim 1, wherein theSOD precursor comprises an organic compound.
 10. The method of claim 9,wherein the SOD precursor comprises poly(methylsil(sesqui)azane). 11.The method of claim 1, wherein the acid comprises a photo acidgenerator.
 12. The method of claim 11, wherein a weight percentage ofthe photo acid generator to the liquid SOD precursor is from about 0.5%to about 20%.
 13. The method of claim 11, wherein a weight percentage ofthe photo acid generator to the liquid SOD precursor is from about 1% toabout 15%.
 14. The method of claim 11, wherein a ratio of the photo acidgenerator to the SOD precursor by weight is from about 0.0025 to about0.01
 15. A method of forming a hard mask resistant to photoresistscumming, comprising: depositing a spin-on dielectric (SOD) precursorcontaining an acid over a substrate; and solidifying the SOD precursorto form an inorganic hard mask layer.
 16. The method of claim 15,further comprising: depositing a photoresist layer over the inorganichard mask layer; exposing a pattern in the photoresist layer; developingthe pattern in the photoresist layer after exposing the photoresistlayer; transferring the pattern to the inorganic hard mask layer afterdeveloping the pattern; and removing the photoresist layer aftertransferring the pattern to the first hard mask layer
 17. The method ofclaim 16, wherein the process is used to form part of an integratedcircuit.
 18. The method of claim 16, wherein no de-scumming step isneeded after removing the photoresist layer.
 19. The method of claim 15,wherein the acid comprises a photo acid generator.
 20. The method ofclaim 19, wherein the photo acid generator comprises a diazoquinone. 21.The method of claim 19, wherein the photo acid generator comprises ahydrogen diazoquinone.
 22. The method of claim 19, wherein the photoacid generator comprises a methyl diazoquinone.
 23. The method of claim19, wherein the photo acid generator comprises a fluoride anion.
 24. Themethod of claim 23, wherein the photo acid generator comprises aphosphorus fluoride anion.
 25. The method of claim 23, wherein the photoacid generator comprises an antimony fluoride anion.